Air arc gouging is a process by which metal is removed. Air arc gouging is used mainly in conjucntion with repair work, such as to remove a bad weld, or finishing work, such as removing the welds off the back side of a joint. In this process, a copper-coated carbon rod is touched to the workpiece, such as a plate, to start an arc. The rod is then backed off slightly so that the rod does not touch the plate as long as the arc is maintained. The arc melts the selected portion of the plate, or the selected portion of the weld on the plate. The carbon rod is held by a gouging assembly, such as an air arc torch assembly, which allows compressed air to flow a high speed down the sides of the rod and blow the molten metal away from the melt area. The process therefore makes a hole if the gouging assembly is stationary or makes a trough if the gouging assembly moves.
The air is typically provided to the gouging assembly at a pressure of 60 to 100 psi. The gouging process requires approximately 460 amps at approximately 42 volts (arc voltage). Very little adjustment of the current or voltage is required.
In the past, a resistor grid has been used to provide the current for the air arc gouging process. The resistor grid contains a plurality of low resistance, high power resistors. The resistor grid works quite satisfactorily but has several disadvantages. The first disadvantage is that the resistor grid is quite large and heavy. This arises from the power that the resistor grid must dissipate. If an 80 volt power supply is used, the arc voltage is 42 volts, and the arc current is 460 amps, it can be quickly determined that the power dissipated by the resistor grid is approximately 17.5 kilowatts, thereby necessitating large resistors so that the resistors do not overheat and burn up. Because of the power dissipation, the resistor grid becomes quite hot and must cool before being moved between jobs. A typical resistor grid is 24 inches long by 12 inches high by 18 inches wide and weighs 65 to 70 pounds. From the size, weight and power dissipation of the resistor grid it will be readily appreciated that the resistor grid is not a conveniently portable device. In ship building and ship repair operations the resistor grid is typically placed outside, on the deck of the ship, and cables are run from the resistor grid to the point where the gouging operation is to be performed. The cables are run in addition to any cables which may be necessary to operate a welding station. When one gouging operation is completed the cables are retrieved and then routed through the ship to the next gouging operation. It will therefore be appreciated that a substantial number of man hours are used just in the process of routing and retrieving the cables and setting up for the gouging operation with the resistor grid. It would seem that the resistor grid could simply be moved to the area where the gouging operation is to be performed but, in practice, this is generally undesirable because of the size, weight, and heat dissipation of the resistor grid.
In addition to the resistor grid, gouging operations are sometimes performed with a high current welding station or power supply, such as a 600 amp STICK welding station or power supply. However, the STICK device also suffers from some of the same problems as the resistor grid: size, weight, and heat generation.
In addition to the problems enumerated above, these devices also suffer from the problems of cost and utility. Except for the gouging operation and pickup STICK welding or tacking the resistor grid essentially has no other utility and, because of the cost of the resistor grid, and the amount of space required to store the resistor grid when it is not being used, the number of resistor grids purchased is usually held to a minimum. However, this minimum number of resistor grids frequently means that one gouging operation, and the subsequent repair, must wait for the completion of the current gouging operation. This means that man hours are often wasted while the welding operator awaits the availability of a resistor grid for gouging.
In addition to the above shortcomings, the resistor grid is also inefficient. That is, because the arc voltage is only 42 volts whereas the power supply voltage is typically 80 volts, approximately one half of the output of the power supply is wasted as heat and is not used for the gouging operation. Also, because of the approximately 460 amp current requirement of a typical gouging operation, the cables between the resistor grid and the area in which the gouging is to be performed must be quite large, such as 3/0 or 4/0 cable.
Also, the resistor grid has a limited short circuit output current capability, typically less than 1000 amps. This limited current capability sometimes causes the rod to "stub out" and overheat the copper sheath if the rod is touched to the plate. In this case the rod should be broken off and the arc restarted to resume smooth gouging. To avoid this problem, two resistor grids are occasionally used in parallel to increase the output current capability and prevent "stubbing out".
Therefore, there is a need for an apparatus for air arc gouging which is lighter and smaller than a resistor grid or a STICK welding station.
Furthermore, there is a need for an apparatus for air arc gouging which is of a size and weight so as to be conveniently portable and which operates from an 80 volt supply.
Furthermore, there is a need for an apparatus for air arc gouging which minimizes the size and number of extra cables that must be run in order to perform a gouging operation.
Furthermore, there is a need for an apparatus for air arc gouging which is more efficient than a resistor grid.
Furthermore, there is a need for an apparatus for air arc gouging which has an output current capacity sufficient to prevent the rod from stubbing out.
Furthermore, there is a need for an apparatus for air arc gouging which is of a size, weight, cost, and electrical efficiency so as to make it economical and practical to obtain and maintain a larger number of these devices than the typical number of resistor grid devices.